Enable Autoconf's TAP driver.

Autoconf's TAP driver is more fully featured and is able to understand a tester (e.g., sass-spec's `sass-spec.rb`) that runs multiple tests. You get a more detailed output of your test results.

For example, see [this Travis build](https://travis-ci.org/QuLogic/libsass/builds/11537833#L377) where each individual test is recognized by autoconf's make rules.

Depends on hcatlin/sass-spec/pull/3.